The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A camshaft actuates valves of an internal combustion engine. In a dual overhead camshaft configuration, the engine includes an exhaust camshaft and an intake camshaft for each bank of cylinders. Rotation of the camshafts actuates intake and exhaust valves of the engine. Position and timing between a crankshaft and the camshafts are adjusted for proper synchronization of intake and exhaust valve events to cylinder piston positioning.
An engine control system may include one or more camshaft phasing devices (camshaft phasors). A camshaft phasor may be used to create a variable rotational offset between the exhaust camshaft and the intake camshaft and/or the crankshaft. The offset alters opening and closing times between intake and exhaust valves.
Engines configured with multiple camshaft phasors can exhibit regions of operation with reduced performance or driveability or increased emissions due to a mismatch between the phasors. This mismatch in phasor performance may refer to a difference in relative velocities between the phasors. The mismatch can contribute to conditions of excessive overlap and high dilution or reduced overlap and low dilution during periods of transition. Overlap refers to when both intake and exhaust values are in an open state during the same time period. Dilution refers to the capturing of diluent gas (exhaust gas) in a cylinder. The mismatched performance may be due to different loading on each of the camshafts.
For example, depending upon whether a phasor is moving in a retarding or advancing direction, the response rate of the phasor may be different due to engine loading on the phasor. As another example, when a torque balance is used on a phasor, such as a return spring, the rate that the phasor responds may be different than a phasor without a torque balance. As a further example, when a device is driven off of one camshaft, such as a fuel pump driven off of an exhaust camshaft, the camshaft responds differently than another camshaft without such loading. As yet another example, the fluid pressure between phasors and/or the supply voltage to phasors may be different. This also results in variability in performance of phasors.
A camshaft phasor based control system typically includes a control valve and a phasor. The control valve is used to adjust passage of hydraulic fluid to the phasor based on a commanded position signal. The flow of hydraulic fluid controls movement of a vane or valve shuttle within the phasor and thus relative positioning between camshafts and/or a crankshaft. Once the valve shuttle is in a commanded (desired) position, fluid flow to and from the control valve is stopped, thereby locking the actuator of the camshaft phasor in a fixed position. This position is referred to as a control hold position.
The positioning of the valve shuttle is achieved by varying the energy supplied to a solenoid which moves the valve shuttle via a control hold duty cycle (CHDC) signal. Typically, the CHDC signal is based on a regression model that is developed during manufacturing of a vehicle. The regression model is developed over time via vehicle testing and post processing of test data. Once developed, the regression model is stored in a camshaft phasor control system of a vehicle and is unchanged. Due to component wear, accuracy of the regression model decreases over time.